cut engine knocks

R

rustychevy

New member
Every so often after I cut the engine it starts knocking and the car shakes along with it. I haven't kept count, maybe once a month or every other month. What is the cause and what should I do when this happens? At the moment I run the engine for a few seconds. Is there anything I need to do as far as maintenance is concerned?
 
How old is the car?

Back when I was a wee lad, this was known as engine run on (I guess it still is), and was caused by upper-cylinder engine deposits (carbon) and low-octane fuel. The fix back then was to use upper-cylinder cleaner (Marvel Mystery Oil, etc.), "blow out the carbon" on the highway, pour water down the carb or any number of other useless techniques. What really worked was higher octane fuel, leaving the car in gear when you shut it off, etc.

This problem went away with fuel injection because the fuel supply is shut off when you turn the ignition off. If you have this problem on a FI car, you may have a leaky injector.
 
What year and type of car is it?

As Setec said, there are a few different things that can cause "dieseling" (engine continuing to run with the key off), but the most common fix on a 70's or 80's carbureted engine was to lower the engine idle speed.

Bill
 
I would want to find out the source of the pre-ignition and eliminate it.

Is the vehicle overheating or if you have a temp gauge, is the water temperature higher than normal ?

If it is easy to do and you have the tools, how about removing all the spark plugs and looking at their condition?
One or more may be fouled enough to be causing this run on or dieseling, etc..

And if you are able to look into each cylinder through the spark plug hole, you might see what could be causing the problem too.
It has to be something that is burning in a cylinder or cylinders after the car is shut off that is wanting it to continue to run like that..
Lots of carbon build up, etc., can cause this too..

I personally like the idea of its an older vehicle of putting in something like marvel mystery oil, warming up the car properly, getting the engine oil up to temps, and going out somewhere safe and running it through the gears.
Just revving it in the driveway sometimes does not work..

Good luck with this !
Dan F
 
You may want to check your ignition timing. If it is advanced too much it will cause excessive cylinder pressure and will contribute to the "dieseling" that you are describing. I know this first hand as myself and my father used to build High Performance street engines. In order to abstract maximum power we would run as much advance in the distributor as possible. When we got to the point of "dieseling," we would then back down on the advance a couple of degrees and it would fix the problem. A higher octane fuel always helped also. The higher octane could withstand increased cylinder pressures without detonating or "dieseling.
 
My car is an 84 Chevy Caprice Classic. I would eliminate the overheating suggestion. The last time it happened it was after a 15 minute return home trip, no more than 55 mph, the last few minutes at 35 mph.
 
rustychevy said:
V8 engine.
Click to expand...
I stand by my original post. The 305 carbureted V8's from that era (same thing I have in my El Camino) are very sensitive to dieseling from elevated idle speed. Make sure all your vacuum lines are intact and haven't hardened from age, causing leaks. Timing is not usually a critical thing; Factory spec is 0 degrees, but I've run mine as high as 6-8 degrees BTDC without harm. There's an idle speed adjustment screw on the lower left side of the carb. Wind it out a turn or two (counter-clockwise) and see if you still have a stable, smooth idle in gear. There's also a solenoid on the left front corner of the carb that increases the idle when the A/C is on. If your problem is showing up only when the A/C is on, unplug the wire to the solenoid and see if that cures it. There are no adjustment provisions on the solenoid plunger, so you'd have to slot the holes in the solenoid bracket.
Hope this helps.

Bill
 
Wow that guy is a Chevy Guru. Also check for carbon build-up in the intake egr area. Car is probably suffering from lack of use. Use a better fuel like Shell V-power. Clean the carbon out of the carb. Is the check engine light on? Bring it up on the interstate and knock it to the wood and clean it out a bit. When I worked at Cadillac we used the top engine cleaner, used to dump it in and stall the car, let it sit overnight. Cleaned the carbon from the piston tops and the like. I know wide open throttles or WOT are recommended for the Northstar Cadillac I drive daily. Most of my cars suffer from a lack of use. I drive them as the weather allows. Also add a bottle of Techron to the fuel and fill the tank to avoid moisture build up over sitting long periods.
 
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